In FIG. 99, (a) is an oblique view of the main parts of a prior-art magneto-optic read-write device as shown, for example, in Preprints of the 34th Joint Congress of Applied Physics, Spring 1987, 28 P-Z L-3; (b) is a sectional view illustrating optical reading and writing of the recording medium; and (c) is a plot of the laser power variations for writing information in areas on the recording medium. In these drawings, 1 is a magneto-optic recording medium comprising a glass or plastic substrate 2, a first magnetic layer 3, and a second magnetic layer 4. An exchange coupling force acts between the first and second magnetic layers 3 and 4, tending to align their magnetization in the same direction. A laser beam is focused by an objective lens 5 onto a spot 6 on the information medium 1. The numeral 7 indicates areas in which the direction of magnetization in the first magnetic layer 3 is upward in FIG. 99 (b), this indicating the recording of binary "1" data. An initializing magnet 9 generates a magnetic field of substantially 5000 oersteds to initialize the second magnetic layer 4. A bias magnet 8 disposed facing the objective lens 5 with the information medium 1 in between generates a magnetic field of substantially 200 to 600 oersteds. In FIG. 99 (c) laser power is shown on the vertical axis and areas are indicated on the horizontal axis. The laser power is modulated to record the information "1" in the region R1 and the information "0" in the region R0. The dash-dot line in FIG. 99 (a) separates new data (DN) on the left from old data (DO) on the right.
The operation will be explained next. The recording medium 1 is rotated in the direction of the arrows in FIG. 99 (a) and (b) by a support and driving mechanism not shown in the drawing. The first magnetic layer 3 has the same properties as the recording layer in the media used in general magneto-optic disks comprising, for example, Tb.sub.21 Fe.sub.79, and here too it functions as a reading and writing layer. The second magnetic layer 4, called the auxiliary layer, comprises Gd.sub.24 Tb.sub.3 Fe.sub.73, for example, and provides the overwrite function, enabling new information to be written over old information in real time. The Curie temperatures Tc1 and Tc2 of the first and second magnetic layers 3 and 4, their room-temperature coercivities Hc1 and Hc2, and their room-temperature exchange coupling strengths Hw1 and Hw2 satisfy the following relations: EQU Tc1&lt;Tc2 EQU Hc1-Hw1&gt;Hc2-Hw2
First the reading of information recorded in the first magnetic layer 3 (the recording layer) will be explained. As shown in FIG. 99 (b), the first magnetic layer 3 is magnetized in the up direction to represent a "1" and in the down direction to represent a "0." When this information is read, the first magnetic layer 3 is illuminated by the beam spot 6, and the magnetic orientation of the first magnetic layer 3 in the beam spot 6 is transformed by the well-known optical Kerr effect to optical information, in which form it is detected. FIG. 100 indicates the temperature changes in the magnetic layers in the spot caused by the laser beam power, with A corresponding to the intensity of the laser beam that illuminates the recording medium 1 during reading. At this intensity the maximum temperature increase in the first and second magnetic layers 3 and 4 in the beam spot 6 does not attain the Curie temperatures Tc1 and Tc2 of these layers, so the illumination in the beam spot does not erase the direction of magnetization; that is, it does not erase the recorded information.
Next the overwriting operation will be explained. The initializing magnet 9 in FIG. 99 generates a magnetic field of intensity Hini in the direction of the arrow b (up) in the drawing. This field Hini is related to the coercivity and exchange coupling strength of the first and second magnetic layers 3 and 4 as follows: EQU Hc1-Hw1&gt;Hini&gt;Hc2+Hw2
As a result, when the information medium 1 revolves in the direction of the arrow a in FIG. 99 (b), those parts of the second magnetic layer 4 that pass over the initializing magnet 9 are uniformly magnetized in the up direction, regardless of the magnetic alignment of the first magnetic layer 3. The first magnetic layer 3 itself is not affected at room temperature by the magnetic field of the initializing magnet or by the exchange coupling force exerted by the second magnetic layer 4, so it remains in its previous state.
To write a "1," which means to magnetize the first magnetic layer 3 in the up direction, the laser beam is modulated to the intensity B in FIG. 100. The temperature in the beam spot 6 then rises above the Curie temperature Tc1 of the first magnetic layer 3, but does not reach the Curie temperature Tc2 of the second magnetic layer4. Consequently, the first magnetic layer 3 loses its magnetization, while the second magnetic layer 4 retains the upward magnetic alignment given by the initializing magnet 9. As the disk turns and the area leaves the illumination of the beam spot 6, when the temperature of the first magnetic layer 3 falls below its Curie temperature Tc1, the magnetic alignment of the second magnetic layer 4 is transferred to the first magnetic layer 3, so that the first magnetic layer 3 becomes magnetized in the up direction, corresponding to a "1."
To record a "0," which means to magnetize the first magnetic layer 3 in the down direction, the laser beam is modulated to the intensity C in FIG. 100. The temperature in the beam spot 6 then rises above both the Curie temperature Tc1 of the first magnetic layer 3 and the Curie temperature Tc2 of the second magnetic layer 4. Consequently, the first and second magnetic layers 3 and 4 both lose their magnetization. As the disk turns and the area leaves the illumination of the beam spot 6, when the temperature of the second magnetic layer 4 falls below its Curie temperature Tc2, the second magnetic layer 4 is magnetized in the down direction by the weak magnetic field applied in the direction of the arrow c (down) in FIG. 99 by the bias magnet 9. Moreover, when the temperature of the first magnetic layer 3 falls below its Curie temperature Tc1, the magnetic alignment of the second magnetic layer 4 is transferred to the first magnetic layer 3, so that the first magnetic layer 3 becomes magnetized in the down direction, corresponding to a "0."
By the above overwriting operations, new information can be written over old information in real time by modulating the laser beam power between the values B and C in FIG. 100 according to the binary codes "0" and "1" of the new information. Since the prior-art magneto-optic recording medium is structured as above, however, it has the problems that an initializing magnet with a strong magnetic field is required and the overall structure of the readwrite apparatus is complex and large in size.
The present invention is directed toward a solution of such problems, an object being to obtain a magneto-optic recording medium that does not require an initializing magnet and can be easily overwritten. Another object is to provide a fabrication method for such a magneto-optic recording medium.